Add `server.conf` default options for Data Tiering / Index Set Defaults Configuration is ignored

## What?
We regularly launch new Graylog deployments as a part of staging environments. Until recently we had used the `server.conf` option `elasticsearch_max_size_per_index` together with `rotation_strategy = size` to set a sensible size for the default index that is automatically created. However now in Graylog 6, the default index set is created with Data Tiering enabled and there's no config entry or env var to control this behavior. Basically #13018 is unusable with this and there's not a single word about it in the [UPGRADING.md](https://github.com/Graylog2/graylog2-server/blob/6.0.4/UPGRADING.md).

## Why?
People want to control the default settings for their index, especially since #13018 was merged. However not only is it not possible to set *legacy* rotation strategies via `server.conf` anymore, there's also no way to override the default of 30d/40d for data tiering. The Index Set Defaults under `https:///system/configurations/Index%20Set%20Defaults` are completely ignored and they don't even offer a Data Tiering option.

## Your Environment

* Graylog Version: 6.0.4
* OpenSearch Version: 2.11.0
* MongoDB Version: 5.0.25
* Operating System: Docker
* Browser version: Chrome 127